1. Field of the Invention
The present invention relates to a droplet discharge-condition detecting unit configured to optically detect a discharge condition of a droplet discharged from a droplet discharger of a droplet-discharging device, such as an ink droplet discharged from a recording head of an inkjet recording device. The present invention also relates to a droplet-discharging device, such as an inkjet recording device, equipped with such a droplet discharge-condition detecting unit.
2. Description of the Related Art
Inkjet recording devices have the following advantages. For example, inkjet recording devices allow recording heads to be made compact, can record high resolution images at high speed, can perform recording on standard paper without any special treatments, require low running costs, have low noise, and can readily perform color-image recording.
However, in inkjet recording devices, there are cases where ink droplets are not discharged from a recording head (which will be referred to as “defective discharge” hereinafter) or the discharge direction is deflected to cause ink droplets to be discharged in an improper direction (which will be referred to as “deflective discharge” hereinafter). For example, the defective discharge and deflective discharge may be caused if the nozzles of the recording head are clogged with dust or thickened ink. The defective discharge and deflective discharge can also be caused if heaters are disconnected in a case where the device is a type that discharges ink droplets by using thermal energy. Additionally, the defective discharge and deflective discharge can also be caused if the nozzle holes are coated with ink droplets. When such defective discharge and deflective discharge occur, a streak-like unevenness may form on a recorded image in a scanning direction of the recording head, thus impairing the quality of the recorded image. Moreover, there is also a case where the rate of discharge of ink droplets (which will be referred to as “discharge rate” hereinafter) becomes lower, which can also lower the quality of a recorded image.
Various techniques for detecting a defective discharge using a light emitting element and a light receiving element have been proposed. One proposed technique is referred to as an optical defective-discharge detecting technique. According to this technique, when an ink droplet is discharged, the ink droplet passes through a light beam emitted from the light emitting element towards the light receiving element so that the ink droplet instantaneously blocks the light beam. This blocking of the light beam by the ink droplet causes the quantity of light received by the light receiving element to change, thereby changing an output from the light receiving element. Consequently, based on the change in the output, it is determined whether or not the ink droplet is discharged. For example, this technique is discussed in Japanese Patent Laid-Open No. 11-192726, and components thereof are shown in FIG. 8.
Referring to FIG. 8, a recording head 8 is provided, and a lower surface of the recording head 8 defines a discharge-nozzle surface 8a. The discharge-nozzle surface 8a is provided with a plurality of discharge nozzles. The recording head 8 is contained in a carriage, not shown. When the carriage moves, the recording head 8 is carried in a direction perpendicular to the page. In a state where the recording head 8 is at a predetermined position within a moving range thereof, a light emitting element 11 and a light receiving element 12 are positioned on opposite sides of an area below the discharge-nozzle surface 8a of the recording head 8, such that the light emitting element 11 and the light receiving element 12 face each other. Diaphragm plates 13′, 14′ are respectively disposed near front surfaces of the light emitting element 11 and the light receiving element 12 that face each other. The diaphragm plate 13′ is provided with a single aperture 13a′, and likewise, the diaphragm plate 14′ is provided with a single aperture 14a′. FIG. 9 illustrates the diaphragm plate 14′ as viewed from a side of the light emitting element 11 and shows an example of a shape and location of the aperture 14a′. In detail, the aperture 14a′ is given a rectangular shape having a predetermined width W (of, for example, about 4 mm) and a predetermined height H (of, for example, about 2 mm). Likewise, the aperture 13a′ is given the same shape and dimension. Furthermore, as viewed from the side of the light emitting element 11, the center of the aperture 14a′ and the center of the light receiving element 12 are aligned with each other. Likewise, the aperture 13a′ and the light emitting element 11 have the same relationship. When light is emitted from the light emitting element 11, a light beam 15 that passes through the apertures 13a′ and 14a′ (which will be referred to as a “detection beam” hereinafter) is received by the light receiving element 12. An optical path of the detection beam 15 extends parallel to the discharge-nozzle surface 8a of the recording head 8.
When performing a discharge-condition detection process, ink droplets are discharged from ink discharge nozzles in the discharge-nozzle surface 8a of the recording head 8 in a direction indicated by an arrow 18, which is perpendicular to the detection beam 15, and the ink droplets instantaneously block the detection beam 15. This changes the quantity of light received by the light receiving element 12, causing an output from the light receiving element 12 to change. The output from the light receiving element 12 is converted to an electric signal as a detection signal. Based on the detection signal, it can be determined whether or not the ink droplets are discharged.
FIG. 10 illustrates a waveform of a detection signal 17 based on the output from the light receiving element 12 and a waveform of a driving signal 16 when each of the nozzles of the recording head 8 is driven at a discharge frequency of 1 kHz.
In FIG. 10, the driving signal 16 is a C-MOS negative logic signal of 3.3 V. When the driving signal 16 decreases to 0 V, the nozzle is driven, thereby starting a discharge operation of an ink droplet from the nozzle. When the discharged ink droplet blocks the detection beam 15, the detection signal 17 is changed (is lowered to approximately −8 V) at a changing point indicated by an arrow 17b. The changing point 17b indicates that the detection beam 15 is blocked by the discharged ink droplet. Based on the presence of the changing point 17b, it can be determined whether or not the ink droplet was discharged.
However, even though it can be determined whether or not an ink droplet is discharged by using the above-referenced technique, the technique does not provide functions for detecting a deflective discharge and a discharge rate.
On the other hand, Japanese Patent Laid-Open No. 2003-276171 discloses an example of an apparatus for detecting a deflective discharge and a discharge rate. Specifically, in this example, a plurality of sets (for example, two sets) of discharge-condition detecting units are provided, each of which is the same as that shown in FIG. 8 and includes the light emitting element, the light receiving element, and the diaphragm plates. The plurality of sets of the discharge-condition detecting units is arranged in parallel to the discharge direction of ink droplets. According to this apparatus, in a case where the discharge direction is deflected as a result of deflective discharge, an ink droplet may block a detection beam of the discharge-condition detecting unit of the first set, but will not block a detection beam of the discharge-condition detecting unit of the second set. Based on this result, a deflective discharge can be detected. Moreover, by measuring the time between a point at which the ink droplet blocks the detection beam of the first set and a point at which the ink droplet blocks the detection beam of the second set, a discharge rate can be determined.
However, the apparatus of Japanese Patent Laid-Open No. 2003-276171 provided with the plurality of sets of discharge-condition detecting units leads to an increase in the cost of components. Moreover, a large installation space is necessary for the plurality of sets of discharge-condition detecting units, which leads to an increase in the overall size of the recording device. Furthermore, it is also required that the distance between the center of the first discharge-condition detecting unit and the center of the second discharge-condition detecting unit onward be equal to or greater than the size of the light emitting elements or the light receiving elements. This implies that the distance between the detection beams of the plurality of discharge-condition detecting units also becomes large, thus lowering the detection accuracy for detecting a deflective discharge. It is possible to reduce the distance between the plurality of sets of discharge-condition detecting units to some extent by using small-size, high-intensity light emitting elements and small-size light receiving elements. However, this is not preferable since small-size, high-intensity light emitting elements are expensive, and small-size light receiving elements have low sensitivity due to having a small light receiving area.